Legally, and in hazardous chemical labeling, poisons are especially toxic substances. Less toxic substances are labeled "harmful", "irritant", or not labeled at all.

In medicine and zoology, toxins and venoms are different from poisons. Toxins are the result of a biological process. Venoms are substances which the organism uses to harm other species. Certain organisms use venoms for hunting, or as a defense. If an organism is poisonous, such as many mushrooms, it is harmful to eat. If it is venomous, like snakes or honeybees, it has a harmful bite or sting. For some very deadly bites humans have developed effective antivenoms.

Often it is only the quantity of a substance that makes the difference. Drinking alcoholic drinks may lead to aggressive behaviour, problems with speech, and different forms of amnesia. This effect is called intoxication. People who drink even more may go into shock. At the same time, alcohol can be used as a disinfectant.

Pollution is also sometimes poisonous. For example, large amounts of potato-peeling scraps or milk can kill organisms in waterways, because they contain so many nutrients for microbes that the microbes consume all oxygen from the waterway.[source?]

In 2010, poisoning resulted in about 180,000 deaths, down from 200,000 in 1990.[5] There were approximately 727,500 emergency department visits in the United States involving poisonings—3.3% of all injury-related encounters.[6]

↑Lozano, R (2012). "Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the global burden of disease study 2010.". Lancet380 (9859): 2095–128. doi:10.1016/S0140-6736(12)61728-0. PMID23245604.